A torsion spring has a coil portion with one or more coils usually forming a generally circular annulus with a coil axis and a transverse diameter generally perpendicular to the coil axis. Some torsion springs may even have less than a full coil. The ends of the coil usually extend from the coil to receive forces that induce torque in the spring. These extensions usually extend in a generally tangential direction but can extend at any angle from the coil or any place about the circumference of the coil depending upon the use to which the torsion spring is put. When a moment is applied to the torsion spring, (a moment being the applied force to the extension multiplied by the distance from the centerline of the coil where the force is applied to the extension), the coil deflects and the material from which it is made is placed under stress. When the moment is relaxed, the coil relaxes and returns to its original shape. As deflection increases, stress increases toward the elastic limit of the material. If the elastic limit is exceeded the coil retains a permanent deflected even after the moment is released. Maximum desired deflection of a coil for a particular applied moment is an important criterion for spring design. In conventional spring designs, one can select material, shape, cross section and the number of coils, among other things, to provide the desired spring performance.
In certain applications, for example, where the force must be applied close to the center line of the coil or where the coil must be used in a confined space so that shape and number of coils are limited, even small deflections can cause large variations in stress at different points around the circumference of the coil. In such applications, portions of the coil can approach the elastic limit even at small deflections.
If conventional torsion springs which have force receiving extensions facing the same side of the coil axis are deflected, high moments and thus high stresses will be experienced on that part of the coil away from the extensions and lower moments and thus lower stresses will be experienced on that part of the coil close to the extensions. The bulk of material in the low stress area may stiffen the coil. The added stiffness in this area can make it harder for this area to deflect. This stiffness can cause deflection in some other area of the coil and possibly increase the tendency of other portions of the coil to approach the elastic limit of the material. We believe that the extra bulk of material in the lower stressed area of the coil can exaggerate the non-uniform stress distribution and actually decrease the effectiveness of the spring. It would be desirable to have a torsion spring with a more uniform stress distribution so that a single torsion spring could be used over a wider variety of angular deflections.
One application of particular interest to us is a torsion spring aneurysm clip where the force receiving extensions face the same side of the coil axis to form the shoulders of aneurysm clip.
An aneurysm is a permanent dilitation of the wall of a blood vessel usually caused by weakening of the wall as a result of some pathological condition. In laymen's terms, the wall weakens and pressure in the vessel causes the wall to expand into a balloon appendage on the side of the vessel. The balloon often has a neck portion extending from the wall and an expanded portion connected to the neck, although an aneurysm may take on various shapes.
One way of treating an aneurysm is to apply a clip to seal off the neck portion of the aneurysm close to the blood vessel wall so that blood pressure will not be exposed to the weakened expanded portion of the aneurysm. Thus, the possibility of the aneurysm bursting is reduced and hopefully eliminated. It is hoped that the clip will seal off the weakened portion of the wall so that the blood vessel can heal.
In the past, torsion spring-type aneurysm clips have been used to effectively seal off certain aneurysms. An aneurysm clip of the kind discussed in the present application is similar to that shown in U.S. Pat. No. 3,827,438 but this patent does not suggest or disclose the special torsion spring discussed in the present patent application. These torsion spring clips have a coil portion with a first arm extending from one end of the torsion spring and offset to one side of the coil and a second arm extending from the other end of the torsion spring offset on the same side of the coil. Each of the first and second arms has a shoulder portion, a cross over portion and a jaw portion. When the shoulder portions are moved toward one another (with a special forcep) so as to torque the torsion spring, the jaw portions move away from each other toward an open position so that the aneurysm can be grasped between the two confronting jaw portions to seal off the aneurysm.
An aneurysm clip designer must deal with several design constraints. Since aneurysm clips are used in the brain and are often permanently implanted they should be as small as possible so that they may be used in very confined spaces. The implant must be biocompatible thus only a few materials, like high strength, high alloy metals are recommended for use. The jaw closing force must be large enough to seal off the aneurysm and not be dislodged or otherwise effected by changes in pressure in the blood vessel. Aneurysm clip jaws also have various shapes, lengths and angles for use with different kinds of aneurysms but the coil spring portions of these aneurysm clips are uniform so as to fit into one sized forcep. It is also desirable that the clip be light weight so as not to pull on the blood vessel to which it is attached.
Thus, the aneurysm clip designer must deal with limits on the clip material, size, weight and force requirements.
An aneurysm clip designer wishes to obtain the maximum jaw deflection from a particular configuration torsion spring. In a conventional torsion spring application, if one wanted larger deflections one would apply the forces at a point farther away from the center line of the coil spring to obtain a higher moment. That is inconvenient for aneurysm clips because it increases the size of the clip and could require the use of different forceps. Some giant aneurysms require clips whose jaws open wider than most available aneurysm clips will permit. Consequently, many surgeons do not attempt to use torsion type aneurysm clips to seal off such giant aneurysms. If one attempts to open the jaws of a conventional clip a sufficient amount to accommodate a giant aneurysm, it is possible for the coil material on the side of the torsion spring away from the jaws to experience stresses beyond its elastic limit and for the bulk of material on the side of the coil close to the jaws to stiffen the torsion spring and exaggerate the uneven stress distribution.
It would be desirable to have a clip which could use the same coil portion but could open wide enough to accept large and small aneurysms without going beyond the elastic limit of the metal of which the clip is made.